1. Field of the Invention
The present invention relates to a solder-free mounting structure for an electronic component, in particular, a mounting structure for an electronic component using a conductive adhesive, and a method for producing the same.
2. Description of the Related Art
In recent years, due to increasing environmental awareness, there have been movements toward elimination of lead-containing solder used for mounting electronic components in the electronic industry.
Under such circumstances, a mounting technique using lead-free solder has been extensively developed, and partially put into practical use. However, the lead-free mounting technique has a number of problems, such as a large effect of an increase in mounting temperature on components with low heat-resistance and difficulty in realizing lead-free electrodes.
On the other hand, mounting using a conductive adhesive has been carried out in circumstances, particularly when restricted to small components such as chip components. More specifically, in addition to the advantage of using no lead, mounting using a conductive adhesive has the following advantages, so that it is expected as a potential technique in the future. First, the treatment temperature of a conductive adhesive is low (about 150xc2x0 C.), compared with that of solder. Second, the specific gravity of a conductive adhesive is almost half that of solder, so that it is easier to render electronic equipment lightweight. Third, unlike solder, a conductive adhesive is not a metal material, so that metal fatigue does not occur, and resistance to repeated stress is outstanding.
Accordingly, by applying a mounting technique using a conductive adhesive to all the components, novel mounting can be expected, which satisfies environmental friendliness and high reliability.
However, a conductive adhesive has adhesion strength smaller than that of solder. Therefore, it is likely to be difficult to replace mounting using solder by mounting using a conductive adhesive. The reason why a conductive adhesive has smaller adhesion strength compared with solder is as follows. Mounting using solder utilizes a metallic bond with electrodes, whereas mounting using a conductive adhesive utilizes physical contact, a hydrogen bond, etc. with electrodes (which are relatively weak).
Conventionally, mounting using a conductive adhesive has been utilized for small components such as chip components. In this case, the mounted components are not likely to receive stress, so that the problem of small adhesion strength has not been raised. However, in particular, when packaged components, etc. (which are rapidly coming into widespread use in recent years) are mounted on printed-wiring substrates, small adhesion strength becomes a serious problem. For example, packaged components such as a Chip Size Package (CSP) and a Ball Grid Array (BGA) are more likely to receive stress compared with chip components. Therefore, there is a high possibility that poor connection occurs in mounting using a conductive adhesive. Stress applied to a mounting structure in which these packaged components are mounted on a printed-wiring substrate is roughly classified into two kinds: shearing stress and bending stress. The shearing stress is likely to be applied to an adhesion portion due to the difference in thermal expansion coefficient between a component and a printed-wiring substrate in the presence of thermal hysteresis. The bending stress is applied to a substrate due to an external force and the like.
Thus, in the case where mounting using a conductive adhesive is applied to all the electronic components, improvement of the adhesion strength becomes important.
In order to improve the adhesion strength of a conductive adhesive, a number of examples of improved adhesive materials have been reported as in JP 59-172571 A. However, it has been difficult to achieve the adhesion strength comparable to that of solder by simply improving adhesive materials.
Hereinafter, an example of a conventional mounting structure obtained by a mounting technique using a conductive adhesive will be described with reference to FIGS. 20 and 21.
Referring to FIG. 20, in the conventional mounting structure, an electrode 93 formed on a substrate 91 is electrically connected to an electrode 94 formed on a substrate 92 via a conductive adhesive 95. Such a mounting structure is obtained by coating the electrode 94 with the conductive adhesive 95, placing the substrate 91 in such a manner that the electrode 93 faces the surface of the conductive adhesive 95, and curing the conductive adhesive 95 by heating.
In the above-mentioned mounting structure, the conductive adhesive 95 is cured while substantially keeping its shape in coating. As a result, as shown in FIG. 21, binder resin 95a is present at the adhesion interface between the conductive adhesive 95 and the electrode 94 in a ratio substantially reflecting its mixture ratio in the conductive adhesive 95. This also applies to the adhesion interface between the conductive adhesive 95 and the electrode 93. Reference numeral 95b in FIG. 21 denotes a metal filler contained in the conductive adhesive 95.
However, the above-mentioned mounting using a conductive adhesive has been hindered from being put into practical use due to the following problems.
First, the adhesion strength in mounting using an adhesive conductive is smaller, compared with that in mounting using solder. One of the reasons for this is that, unlike mounting using solder, a fillet cannot be formed around adhesion interfaces by mounting using an adhesive conductive.
For example, in the conventional mounting structure shown in FIGS. 20 and 21, no binder resin 95a adheres to the side surfaces of the electrodes 93 and 94 and the peripheral portions thereof on the substrates 91 and 92. Accordingly, compared with molding using solder that enables a fillet to be formed around adhesion interfaces, adhesion strength is smaller in mounting using a conductive adhesive. Therefore, the adhesion portion obtained by mounting using a conductive adhesive is likely to be broken by an external force or heat stress, resulting in low adhesion reliability.
Second, in mounting using a conductive adhesive, electrodes are electrically connected to each other by way of contact with a filler in a conductive adhesive. Therefore, the connection resistance is greatly influenced by the state of contact.
For example, in the conventional mounting structure shown in FIG. 21, the binder resin 95a in the conductive adhesive 95 also is present at the adhesion interfaces between the conductive adhesive 95 and the electrodes 93 and 94 in a ratio substantially reflecting its mixture ratio in the conductive adhesive 95. Therefore, a large amount of the binder resin 95a at the adhesion interfaces may adversely affect the electrical connection, which renders the connection resistance high or unstable.
Thus, the conventional mounting using a conductive adhesive has been difficult to be put into practical use as an alternative to mounting using solder, due to insufficient reliability at the adhesion interfaces.
Therefore, with the foregoing in mind, it is an object of the present invention to provide a mounting structure in which adhesion strength and adhesion reliability are improved without using solder.
It is another object of the present invention to provide a mounting structure with high adhesion strength and low and stable connection resistance by improving adhesion reliability between a conductive adhesive and a metal electrode in solder-free molding using a conductive adhesive.
In order to achieve the above-mentioned objects, the first mounting structure of the present invention includes a first electrode and a second electrode electrically connected to each other via a conductive adhesive, wherein a periphery of an adhesion portion between at least one of the electrodes and the conductive adhesive is covered with an electrical insulating layer.
The second mounting structure of the present invention includes a conductive adhesive and a metal electrode electrically connected to each other, wherein the conductive adhesive contains a conductive filler and binder resin, a content of the conductive filler in the conductive adhesive is high in an adhesion portion between the conductive adhesive and the metal electrode, and a part of the binder resin flows and adheres to a periphery of the metal electrode.
According to the present invention, a method is provided for producing a mounting structure for an electronic component including a conductive adhesive and metal electrodes electrically connected to each other. The method includes: coating the metal electrodes with a solvent that dissolves binder resin in the conductive adhesive; coating the solvent with the conductive adhesive; and curing the conductive adhesive by heating so as to connect the metal electrodes to the conductive adhesive.
The first mounting structure of the present invention allows adhesion strength (which used to be small in conventional mounting using a conductive adhesive) to be remarkably improved by covering an adhesion portion with an electrical insulating layer, and hence, contributes to achievement of a solder-free mounting technique using a conductive adhesive.
The second mounting structure of the present invention allows adhesion strength between a conductive material contained in a conductive adhesive and a metal electrode to be enhanced, and low and stable connection resistance to be obtained. Furthermore, since binder resin adheres to the side surface of the metal electrode to form a fillet, a mounting structure with high adhesion strength can be obtained.
Furthermore, according to the production method of the present invention, in the step of curing a conductive adhesive by heating, binder resin contained in a portion of the conductive adhesive that is in contact with a solvent layer dissolves in the solvent layer and flows along an adhesion interface due to an interface effect. Consequently, the content of the binder resin contained in the conductive adhesive becomes low at the adhesion interface between the metal electrode and the conductive adhesive. Therefore, the adhesion strength between the conductive material contained in the conductive adhesive and the metal electrode is enhanced, whereby a mounting structure with lower and more stable connection resistance can be obtained, compared with a mounting structure obtained by a conventional method in which a metal electrode is directly connected to a conductive adhesive without using a solvent. Furthermore, the binder resin that is extruded along the adhesion interface adheres to the side surface of the metal electrode to form a fillet, so that adhesion strength can be remarkably enhanced, compared with the mounting structure obtained by the conventional method.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.